PORTLAND, Ore. — Oregon wildlife officials have started killing sea lions that threaten a fragile run of winter steelhead in the Willamette River.The state Department of Fish and Wildlife obtained a federal permit in November to kill up to 93 California sea lions annually below Willamette Falls south of Portland, Oregon Public Broadcasting reported Wednesday.Officials have so far killed three of the animals using traps they used last year to relocate the sea lions.The sea lions have learned that they can loiter under the waterfall and snack on the vulnerable steelhead as they struggle to make their way over the falls and back into the streams where they hatched in order to spawn.The animals breed each summer off Southern California and northern Mexico, then the males cruise up the Pacific Coast to forage. Hunted for their thick fur, the mammals' numbers dropped dramatically but have rebounded from 30,000 in the late 1960s to about 300,000 today because of the 1972 Marine Mammal Protection Act.With their numbers growing, the dog-faced pinnipeds are venturing ever farther inland on the watery highways of the Columbia River and its tributaries in Oregon and Washington — and their appetite is having disastrous consequences, scientists say.In Oregon, the sea lions are intercepting protected fish on their way to spawning grounds above Willamette Falls, a horseshoe-shaped waterfall about 25 miles (40 kilometers) south of Portland. Last winter, a record-low 512 wild winter steelhead completed the journey, state wildlife officials say.Less than 30 years ago, that number was more than 15,000, according to state numbers.The sea lions were eating so many winter steelhead at the falls that certain runs were at a high risk of going extinct, according to a 2017 study by the state Department of Fish and Wildlife.The state now plans to kill about 40 sea lions that frequent the falls by May, said Bryan Wright, a program manager for the department. The number of sea lions that return to the river should drop to single digits following the killings, he said."The only fish in the river right now are the winter steelhead," Wright said. "If we can remove all these sea lions right now that will be a huge benefit to them."Wildlife officials moved about a dozen sea lions to the coast near Newport, but the animals ended up swimming back to Willamette Falls. Officials determined relocation was not a long-term solution."Unfortunately, that didn't work," Wright said. "But one benefit of that work was that we figured out how to safely capture them, move them, transfer them and figure out all the logistics. Because of that effort we were able to quickly pick up where we left off and start the lethal removal program using the same techniques."According to the Nov. 14 letter from the federal government authorizing the lethal removals, the sea lions targeted for killing must have been observed eating at least one steelhead between Willamette Falls and the mouth of the Clackamas River between Nov. 1 and Aug. 15 or have been observed in the same stretch of river on two consecutive days in that period.The animals will be "humanely euthanized" by a method agreed upon by a special committee established for that purpose. The group includes a veterinarian.Before an animal is killed, the state must alert the National Marine Fisheries Service to see if there is a zoo, aquarium or academic institution that wants the animal. If so, Oregon wildlife managers must hold the sea lion for 48 hours before killing it.Oregon and Washington have already killed more than 150 sea lions below the Bonneville Dam to protect threatened and endangered salmon on the Columbia River.Congress passed a bill last month that will make it easier for states and tribes to kill sea lions, but that bill hasn't taken effect yet.